The objectives of this research proposal are to determine factors which control prolactin concentration in serum of cattle by influencing prolactin release from the pituitary or clearance from serum. To meet the first objective we will conduct experiments to evaluate the effect of gonadal steroids and thyroid hormones on baseline serum prolactin levels and on magnitude of prolactin release by a standard challenge of thyrotropin releasing hormone (TRH). Complementary studies will be carried out in vitro using bovine pituitary cell cultures; we will investigate effects of gonadal steroids, throid hormones, prostaglandins and hypothalamic extracts. Experiments are also designed to determine the effect of tactile and environmental (temperature and light) stimuli on prolactin secretion. To investigate the physiological significance of the prepartum prolactin surge in cattle, we will compare average milk production of cows in which this surge was blocked with ergocryptine to that of cows treated with ergocryptine but receiving exogenous prolactin. To investigate the cause of the prepartum increase in serum prolactin we will administer gonadal steroids to ovariectomized heifers to simulate prepartum changes in serum concentration which normally occur, then quantitate changes in baseline serum prolactin levels and magnitude of prolactin response to TRH. Metabolic clearance rate will be quantitated in cattle during various physiological states and under various environmental conditions to determine if clearance is an important determinant to control of serum prolactin concentration.